Аннотация:One of the large technological problems for the cryogenic version of the OGRAN setup is to
keep mirror’s optical characteristics invariable at low temperature under illumination of the 0.1-1
W laser power. At such power level, the thermal induced lenses in the laser spot on the mirrors
can have significant influence. The mirror’s substrates should have low thermal noises to achieve
high sensitivity and good thermo conductivity at low temperatures to dissipate absorbed power.
At present, sitall, sapphire and calcium fluoride (CaF2) are considered as promising materials for
mirror’s substrates at the wave length 1mkm. In this talk we describe experiments in measuring
of evolution of the integral characteristics of FP cavities such as the finesse and contrast of output
interference pattern under process of FP cavity cooling from 300 K down to 6 K. It resulted in the
final recommendation in favor of fluoride calcium mirrors.